Purchase Orders and Three-Way Matching

Why customers use purchase orders

A purchase order (PO) is a document the customer raises to authorise a purchase before it happens. It specifies what they are buying, the agreed price, and a unique PO number. For larger and government buyers, the PO is the internal permission slip: it confirms the spend was approved by the right person and budgeted for before any commitment was made.

For you, the supplier, this changes the rules of engagement. The customer's accounts team will generally only pay an invoice that ties back to a valid PO. An invoice without one has no authorising document behind it, so from their side there is nothing to pay against. Understanding this up front turns a potential payment blocker into a routine formality.

How three-way matching works

Three-way matching is the control many larger organisations use before releasing payment. Their system compares three documents: the purchase order (what was approved), the goods receipt or delivery record (what actually arrived or was delivered), and your invoice (what you are billing). Payment is released only when all three agree on quantity and price.

The logic is sound from the buyer's perspective: it stops them paying for things they did not order, did not receive, or were overcharged for. The implication for you is precise — your invoice must match the PO and the delivery exactly. A discrepancy in quantity, price or description anywhere across the three documents halts the match and parks your invoice until someone resolves it.

Why a missing PO stalls you

The most common reason an invoice to a large customer goes unpaid is mundane: it has no PO number, or the wrong one. Without a PO reference, the matching system has nothing to compare against, and the invoice falls into an exceptions queue to be chased up manually — which can take weeks, through no bad faith on anyone's part.

The defence is simple discipline. Obtain the PO number before you start the work or deliver the goods, not after. If a customer asks you to begin without a PO, treat that as a flag: get the PO raised first, or at minimum get written authorisation, because billing into a PO-driven process with no PO is how invoices disappear into limbo.

Matching your invoice and handling change

To pass the match, mirror the PO on your invoice. Quote the PO number prominently. Describe the goods or services in the same terms the PO uses, with the same quantities and the same prices. The closer your invoice reads to their PO, the more likely it clears automatically without a human ever touching it, and a consistent invoice template with a clear field for the PO number helps you capture it every time. Small mismatches — a different unit description, a price that drifted — are exactly what stops the match.

Variations need care in a PO world. If the scope or price changes, the customer usually has to amend the PO or raise a new one before you can validly bill the difference. Do not simply invoice extra against the original PO and hope — it will fail the match. Agree the change, get the PO updated, then bill against the corrected figure so your invoice and their authorisation line up.